In order to enable the transmission of optical signals at rates exceeding a few dozen gigabits per second (Gbps), much research has been done in the area of pulse shaping and materials fabrication. In particular, it has been found that traditional bandwidth limitations imposed by chromatic dispersion can be overcome by fabrication of the optical transmission medium such that the induced chromatic dispersion is a function of pulse amplitude. If, in addition, the pulse shape and amplitude are carefully chosen, then an original pulse will maintain the chosen shape and amplitude as it travels along the fiber. Such specially shaped pulses are known as solitons and can be transmitted at intervals as low as 10 picoseconds (ps) between pulses.
In an ideal soliton transmission system, each soliton is at the center of the corresponding symbol interval. Unfortunately, in a practical system, timing jitter influences the position of a soliton relative to the center of the symbol interval and an error occurs when the pulse is shifted too far off center. The timing jitter has three main sources: Gordon-Haus jitter due to the addition of optical noise from upstream optical amplifiers, soliton-soliton interaction from the symbol pattern and interaction between channels of different wavelengths sharing the same fiber in a wavelength division multiplexed (WDM) system. A discussion of timing jitter can be found in the summary paper "Soliton WDM Transmission" by Bruce M. Nyman and S. G. Evangelides, presented at the conference on optical fiber communication (OFC) in 1995 and incorporated by reference herein.
To overcome the error-inducing effects of timing jitter in practical systems, it is necessary to install very high bit-rate regeneration units every five hundred kilometres or less. Since full regeneration units are relatively expensive components, it would be desirable to increase the distance which could be travelled by solitons without requiring their full regeneration. One possible approach is to place partial regeneration units, which are considerably less expensive than full regeneration units, at various points along the optical path.
One known method of partially regenerating solitons is described in an article entitled "Optical Retiming Regenerator Using 1.5 um Wavelength Multielectrode DFB LDs" by M. Jinno and T. Matsumoto, which appeared in Vol. 25, No. 20 of the journal "Electronics Letters", published on Sep. 28.sup.th, 1989 and incorporated by reference herein. The approach disclosed therein is based on a self-pulsating circuit to extract an optical clock from an input signal; the input signal is then combined with the clock signal and injected into a bistable multi-electrode distributed feedback laser diode which then reproduces the original optical data, retimed using the extracted clock. While this method possesses some desirable features, it is limited to a frequency range below 200 MHz and it is not feasible to construct such a circuit to operate at frequencies that are higher by several orders of magnitude.
Another relevant technique involves the use of an electro-absorptive modulator with a recovered clock, as described in WIPO International Patent Application WO 96/27956, published Sep. 12.sup.th, 1996, which is incorporated by reference herein. While the clock recovery system disclosed therein provides re-centering of optical symbols in a desired manner, it is extremely difficult to implement the modulator and the clock recovery apparatus at bit rates on the order of 100 Gbps or more.